I was wondering if I could get perfect extraction timing by measuring the light transmittance (of course with the same coarseness of grind) like a Beer's law sort of thing. Anyone have any readings? Or like a method you use to determine optimal extraction?
Refractometers are widely used to measure the opaqueness of the coffee during brewing to timely determine the extraction time.
Spectrophotometers are used to measure color instead of opaqueness. They are mostly useful to determine the roast degree.
So, why not?
However, in my opinion, perfect is something defined by personal preference when considered with coffee.
If you want to find the perfect roast degree, you may choose your specific beans first, specific brewing method, etc. Then choose the specific roast degree. After that, a spectrophotometer may help you to find that exact point each time you repeat the roasting process. Similar to that, during brewing, a refractometer may help you to find that previously decided specific point each time you repeat that brewing process.
In summary, there is no guidance you can hear as the perfect point that you can scientifically lean on. But you may hear some advice based on the coffee community's common experience. Generally, if you are not into industrial production, you don't generally need nerdy equipment to follow such advise.
I know it's down a different line (I like the idea of using light though) but the most common way to test proper extraction is to perform a TDS (Total Dissolved Solids) test. This is the industry standard.
I don't believe handheld meters to be too expensive but not sure.